Compiling assembly with C is actually pretty easy, you can even get pre-processor commands into your assembly so you can re-use constants between the two.
Situation
While writing my microkernel, it was necessary to add a machine specific context switch, for the ARM-920T processor. This meant I needed to add assembly to my C code.
I also needed to have my user tasks call syscall functions, which ended up being stubs for the swi ARM assembly command. Thus I needed assembly there too, however it needed to share the syscall_id value between the kernel syscall handler, and the user syscall stubs.
How to create the code
Header files
First comes the assemble helpers that will let us add immediate values using standard pre-processor constants.
// asm.h
#define immed(val) # ## val
It simply defines adds a # in front of the value (since ARM assembly uses a hash sign to mark immediate values)
Then define the function headers, so your C compiler knows that the functions will exist when you call the linker later.
// syscall.h
int Create(int priority, void (*code) ( ));
void Exit(void);
Finally we have our shared syscall_id definitions.
#define SYSCALL_ID_CREATE 1
#define SYSCALL_ID_EXIT 12
Assembly
You can see the immed(SYSCALL_ID_CREATE) doing its job to put the value properly into assembly
@ sycall.S
#include <machine/asm.h>
#include <kern/syscall_id.h>
@-----------------------------------------------------------
.text
.align 2
.global Create
.type Create, %function
Create:
@ args = 0, pretend = 0, frame = 8
@ frame_needed = 1, uses_anonymous_args = 0
@ Push the args
stmfd sp!, {r0-r1}
@ Software interrupt, doing the syscall
swi immed(SYSCALL_ID_CREATE)
@ Fix the stack pointer
add sp, sp, #8
@ Exit back to previous frame
mov pc, lr
.size Create, .-Create
@-----------------------------------------------------------
@-----------------------------------------------------------
.text
.align 2
.global Exit
.type Exit, %function
Exit:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 1, uses_anonymous_args = 0
@ Software interrupt, doing the syscall
swi immed(SYSCALL_ID_EXIT)
@ Exit back to previous frame
mov pc, lr
.size Exit, .-Exit
@-----------------------------------------------------------
How to set-up your Makefile
First we need to find each of the files, remember the *.S files are your assembly + preproccessor.
Then we convert their endings to the resulting files that we want: *.o files
# Kernel libraries
ASMLIB = $(shell find ./kern/asm/ -name "*.S")
CLIB = $(shell find ./kern/ -name "*.c")
all: $(patsubst %.c,%.o,$(LIBS)) $(patsubst %.S,%.o,$(ASMLIB))
Then we have the actual commands that will compile the files.
Notice that the *.S files only get pre-processed by gcc (using the -E flag). Then they follow the standard chain to become *.o files.
%.s: %.c
$(CC) -S $(CFLAGS) -o $@ $<
%.o: %.s
$(AS) $(ASFLAGS) -o $@ $<
kern/asm/%.s: kern/asm/%.S
$(CC) -E $(CFLAGS) -O0 -o $@ $<